1. Field of the Invention
The present invention pertains to the field of gyroscopic devices and more particularly concerns stable subsynchronous drive systems for driving the rotors of gyroscope spin motors.
2. Description of the Prior Art
Gyroscopes, especially those of the rate of turn type, typically use hysteresis synchronous motors so that the rotor may be driven at a known constant speed. Such operation is desired where an output of the gyroscope is a precise calibrated output only for a given constant rate of rotation of the gyroscope rotor. However, the standard hysteresis synchronous motor has an inherent property of random synchronization. The seat of this property lies in the fact that each time the motor is started and subsequently synchronizes, the permanent magnetic poles of the hysteresis ring of the motor are re-formed. Further, each time they are newly generated, their locations randomly shift and so do their magnitudes, all apparently under the influence of the revolving magnetomotive force in the ring. Thus, the magnetic axis as established in the rotor in its last preceding operating period will normally not be in alignment with the revolving field when the motor is subsequently started and a new synchronizing point accordingly materializes.
The foregoing random readjustment can induce undesired differences in the magnitude and phase of motor-induced vibrations and can alter the magnetic coupling between the spin motor stator and the gyroscope rotor. The effects of the latter problems are of special concern for flexure-supported free rotor gyroscopes wherein the gyroscope rotor is articulated with respect to the spin motor stator.
It is understood that one attempt has been made, but with minor success, to overcome the foregoing problem by periodic high frequency interruption of the spin motor supply to use an averaging effect of many rapid random resynchronizations. The noise introduced by the method is detrimental to the achievement of low random drift; also, rotor slip caused by the periodic non-synchronous operation of the gyroscope rotor induces uncertainty in the rotor speed, an uncertainty that cannot be tolerated in calibrated inertial systems. Another suggested method was to use a periodic advance or retreat of the spin motor magnetic field which causes remagnetization of the hystereis ring without the periodic rotor speed change caused by the power interruption technique. Although there appears to be a beneficial averaging feature also in this latter technique, it does not overcome rotor speed uncertainty and it is therefore not suited for use in precision applications.